1. Field of the Invention
The present invention relates to a projection exposure apparatus for use in a lithography step in the course of manufacturing a semiconductor element, a liquid crystal display element, etc.
The present invention relates to a projection exposure method and a projection exposure apparatus for use in transfer-exposure of a mask pattern onto a photosensitive substrate when, for example, a semiconductor element, a liquid crystal display element, or the like, is manufactured by a lithography process, and in particular, to a projection exposure method and apparatus for effecting an exposure by switching the step-and-repeat method with the step-and-scan method.
2. Related Background Art
This kind of projection exposure apparatus has hitherto been classified roughly into two types. One of them may involve the use of a method of exposing a photosensitive substrate such as a wafer, a plate, etc. by a step-and-repeat method through a projection optical system having an exposure field capable of including a whole pattern of a mask (reticle). The other type may involve the use of a scan method of effecting the exposure with a relative scan performed under mask illumination of arched slit illumination light, wherein the mask and the photosensitive substrate are disposed in a face-to-face relationship with the projection optical system interposed therebetween.
A stepper adopting the former step-and-repeat exposure method is a dominant apparatus in the recent lithography process. The stepper exhibits a resolving power, an overlap accuracy and a throughput which are all higher than in an aligner adopting the latter scan exposure method. It is considered that the stepper will continue to be dominant for some period from now on into the future.
By the way, a new scan exposure method for attaining a high resolving power has recently been proposed as a step-and-scan method on pp.424-433 of Optical/Laser Microlithography II (1989), SPIE Vol.1088. The step-and-scan method is a combined version of the scan method of one-dimensionally scanning the wafer at a speed synchronizing therewith while one-dimensionally scanning the mask (reticle) and a method of moving the wafer stepwise in a direction orthogonal to a scan-exposure direction.
FIG. 1 is an explanatory view showing a concept of the step and scan method. Herein, shot regions (one chip or multi-chips) arranged in an X-direction on a wafer W are scan-exposed by beams of arched slit illumination light RIL. The wafer W is stepped in a Y-direction. Referring to the same Figure, arrows indicated by broken lines represent a route of the step and scan (hereinafter abbreviated to S and S) exposure. The shot regions undergo the same S and S exposure in the sequence such as SA1, SA2, . . . SA6. Subsequently, the same S and S exposure is performed on the shot regions in the sequence such as SA7, SA8, . . . SA12 arranged in the Y-direction at the center of the wafer W. In the aligner based on the S and S method disclosed in the above-mentioned literature, an image of the reticle pattern illuminated with the arched slit illumination light RIL is formed on the wafer W via a xc2xc reduction projection optical system. Hence, an X-directional scan velocity of the reticle stage is accurately controlled to a value that is four times the X-directional scan velocity of the wafer stage. Further, the reason why the arched slit illumination light RIL is employed is to obtain such advantages that a variety of aberrations become substantially zero in a narrow (zonal) range of an image height point spaced a given distance apart from the optical axis by using a reduction system consisting of a combination of a refractive element and a reflex element as a projection optical system. One example of such a reflex reduction projection system is disclosed in, e.g., U.S. Pat. No. 4,747,678.
Proposed in, e.g., Japanese Patent Laid-open Application No. 2-229423 (U.S. Pat. No. 4,924,257) is an attempt to apply a typical projection optical system (full field type) having a circular image field to an S and S exposure method other than the above-described S and S exposure method which uses the arched slit illumination light. The following are particulars disclosed in this Patent Laid-open Application. Exposure light with which the reticle (mask) is illuminated takes a regular hexagon inscribed to a circular image field of a projection lens system. Two face-to-face edges of the regular hexagon extend in a direction orthogonal to the scan-exposure direction. It is thus attained the S and S exposure exhibiting a more improved throughput. That is, this Patent Laid-open Application shows that the scan velocities of the reticle stage and of the wafer stage can be set much higher than by the S and S exposure method using the arched slit illumination light by taking an as large reticle (mask) illumination region in the scan-exposure direction as possible.
According to the above-described prior art disclosed in Japanese Patent Laid-open Application No. 2-229423, the mask illumination region is enlarged in the scan-exposure direction to the greatest possible degree. This is therefore advantageous in terms of the throughput.
By the way, there is nothing but to take the zigzag S and S method shown in FIG. 1 even in the apparatus disclosed in the above-mentioned Patent Laid-open Application in consideration of actual scan sequences of mask stage and the wafer stage.
The reason for this is given as follows. A diameter of the wafer W is set to 150 mm (6 inch). When trying to complete the exposure of one-row shot regions corresponding to the wafer diameter by only one continuous X-directional scan, the premise is that a ⅕ projection lens system is employed. Based on this premise, a scan-directional (X-directional) length is as long as 750 mm (30 inch). It is extremely difficult to manufacture this kind of reticle. Even if such a reticle can be manufactured, a stroke of the reticle stage for scanning the reticle in the X-direction requires 750 mm or more. Therefore, the apparatus invariably highly increases in size. For this reason, there is no alternative but to perform the zig-zag scan even in the apparatus disclosed in the above-mentioned Patent Laid-open Application.
It is therefore required that the periphery of the pattern region on the reticle be widely covered with a light shielding substance so as not to transfer the reticle pattern within an adjacent shot region with respect to, e.g., the shot regions SA1, SA12 shown in FIG. 1.
FIGS. 2A and 2B each illustrate a layout of a hexagonal illumination region HIL, a circular image field IF of the projection lens system and a reticle R during a scan exposure. FIG. 2A shows a state where the hexagonal illumination region HIL is set in a start-of-scan position on the reticle R. Only the reticle R one-dimensionally moves rightward in the same Figure from this state. FIG. 2B illustrates a state at the end of one scanning process.
Referring to FIGS. 2A and 2B, the symbols CP1, CP2, . . . CP6 represent chip patterns formed in row in the X-direction on the reticle R. A row of these six chip patterns correspond to the shot regions to be exposed by one scanning process in the X-direction. Note that in the same Figures, the central point of the hexagonal illumination region HIL coincides substantially with the center of the image field, i.e., an optical axis AX of the projection lens system.
As obvious from FIGS. 2A and 2B, the light shielding substance equal to or larger than at least a scan-directional width dimension of the hexagonal illumination region HIL is needed for the exterior of the pattern region in the start- and end-of-scan areas on the reticle R. Simultaneously, a scan-directional dimension of the reticle R itself also increases. An X-directional moving stroke of the reticle stage is also needed corresponding to a total of an X-directional dimension of the entire patterns CP1-CP6 and a scan-directional dimension of the hexagonal illumination region HIL. Those are thinkable problems in terms of shaping up an apparatus.
Also, since being optimized for either the step-and-repeat method or the step-and-scan method, the prior-art projection exposure apparatus unavoidably has disadvantages of each of the methods. The disadvantages belonging to the two methods are described in the following.
A. step-and-repeat method
1. In order to increase an area for patterns to be transferred on the reticle, it is necessary to increase a lens diameter of the projection optical system. Thus, the increase of the area is limited together when the manufacturing cost of the projection optical system increases.
2. Since an exposure field to be effected by the projection optical system is in the shape of a square substantially inscribed to an effective exposure field, a distortion of said exposure field becomes larger and an overlap accuracy is deteriorated when the exposure is effected on a layer having a different wafer by use of a different projection exposure apparatus (matching).
3. since the area of an exposure field to be exposed simultaneously is large and an exposure energy (a degree of illuminance) per unit area is small, it is necessary to prolong the exposure time when a resist having a low sensitivity is used, whereby a throughput is decreased.
B. step-and-scan method
1. Though the projection optical system can be manufactured at low cost, the manufacturing cost of a stage mechanism becomes high since it is necessary to scan the reticle and the wafer in synchronization. Moreover, when a resist having a high sensitivity is used, it is necessary to shorten the exposure time. For this reason, the scan velocity of the reticle stage is required to be higher. As a result, the manufacturing cost increases.
2. Due to vibration at the scan-exposure time and an averaging of the distortions in the projection optical system, the image forming performance is deteriorated.
3. When an overlap exposure is effected on different layers on the wafer by use of a single projection exposure apparatus, a distortion becomes different for each exposure. As a result, the overlap accuracy is deteriorated.
It is a primary object of the present invention, which has been devised in view of the foregoing problems, to provide a projection exposure apparatus by a scan method (or an S and S method) exhibiting an increased throughput by minimizing a moving stroke of a reticle stage during a scan-exposure without providing a specially wide light shielding substance along the periphery of a pattern exposure region on a reticle (mask).
To accomplish this object, according to one aspect of the present invention, there is provided a projection exposure apparatus by a scan-exposure method, including an illuminating means for illuminating a mask transfer region with illumination light for an exposure through an aperture of a variable field stop disposed in a position substantially conjugate to the mask. This apparatus also includes a driving means for configuring the aperture of the variable field stop in a rectangular shape (having edges orthogonal to a direction of the scan-exposure) and simultaneously making variable a width of the rectangular aperture of the stop in a widthwise direction (the scan-exposure direction) of the transfer region (pattern forming region) on the mask.
The projection exposure apparatus further includes a control means for controlling the driving means to change a width of the rectangular aperture of the variable field stop in interlock with variations in position of the variable field stop on the mask transfer region which varies due to the one-dimensional movements of the mask stage.
Based on the conventional scan-exposure method, the mask is irradiated with the illumination light via an aperture in a fixed shape (hexagon, arched illumination area, etc.). According to the present invention, however, the scan-directional width of the aperture (variable field stop) is varied interlocking with a scan of the mask or the photosensitive substrate. The same S and S exposure method can be therefore realized simply by sequentially narrowing the aperture width without causing a large overrun of the mask in the start- and end-of-scan areas on the mask. Accordingly, the overrun of the mask stage is eliminated in terms of its necessity or extremely reduced, whereby the moving stroke of the mask stage can be minimized. At the same time, the width of the light shielding substance formed along the periphery of the pattern forming region on the mask may also be small to the same extent as that in the conventional mask. The advantage lies in a decrease in labor for inspecting a pin hole defect in the light shielding substance (normally, a chrome layer) during a manufacturing process of the mask.
Further, the aperture of the variable field stop is set in a shape adapted to the pattern forming region on the mask, thereby making it possible to utilize the apparatus also as a stepper equal to the conventional one.
Besides, an aperture position and a geometrical configuration of the variable field stop are set to cause variations one-dimensionally, two-dimensionally or in a rotational direction within the image field of the projection optical system. It is thus feasible to instantaneously correspond to mask patterns of a variety of chip sizes.
As explained above, according to the present invention, it is possible to minimize the moving stroke of the mask (reticle) in accordance with the scan-exposure method. A dimension of the light shielding band on the mask can also be reduced.
At the same time, the scan-directional illumination region on the mask can be taken large, and, therefore, the throughput can be remarkably enhanced in combination with a diminution in the moving stroke.
It is another object of the present invention, which has been devised in view of the foregoing problems, to provide a projection exposure method capable of enjoying the advantages of the step-and-repeat method and the step-and-scan method and capable of compensating the disadvantages of the step-and-repeat method and the step-and-scan method, as well as a projection exposure apparatus which can be used in embodying such a projection exposure apparatus.
To accomplish this object, according to the present invention, there is provided a projection exposure method which has a step-and-repeat mode and a step-and-scan mode, to effect an exposure in either the step-and-repeat mode or the step-and-scan by using at least one of information pieces on a layout of a plurality of shot regions on a photosensitive substrate, a quantity of integrated exposure required on the photosensitive substrate, configurations of these shot regions, a resolving power required for pattern images of a mask, and an allowance for distortions. Therefore, it is possible to realize an exposure method which can make the most of only the advantages of both the step-and-repeat mode antd the step-and-scan mode, and is excellent in terms of all the performances including the throughput (the number of wafers to be processed per unit time) and the image forming performance, etc.
According to the projection exposure apparatus of the present invention, it is possible to use the above-mentioned exposure method.
According to the present invention, one of the both exposure methods is selected in one of the following manners.
1) An exposure time for one photosensitive substrate is calculated on the basis of a layout of the shot regions, required quantity of integrated exposure, etc. Then, an exposure method having the shorter exposure time is selected.
2) When a configuration of the shot region exceeds the width of an effective exposure field of the projection optical system with respect to a scan direction in the step-and-scan mode, the step-and-scan mode is selected.
3) An exposure mode which can satisfy both the resolving power required for an exposure of mask patterns and an allowance for distortions is selected.
When, for example, an exposure is effected in the step-and-scan mode for each shot region on the photosensitive substrate, if movements among the shot regions are conducted in a direction orthogonal to the scan direction, as indicated by a locus, the exposure time is reduced. On the other hand, when the exposure for each shot region is effected in the step-and-repeat mode, the movements among the shot regions are conducted in the short-side direction, as indicated by a locus. Then, the exposure time is reduced. Therefore, a stepping direction of the photosensitive substrate is switched over in accordance with a selected exposure mode, whereby the exposure time is further reduced.
Moreover, in order to make uniform a distribution of luminance on the mask, it is preferable to dispose an optical integrator in an illumination optical system. In this case, since a cross-sectional configuration of an optical element of the optical integrator is substantially the same as that of an illumination region on the mask, if the exposure mode is switched over to change the configuration of the illumination region on the mask, the optical integrator equipped with an optical element having a cross-sectional configuration substantially equal to the configuration of said illumination region is used to improve the illumination efficiency.
According to the present invention, an exposure is effected in the step-and-scan mode or the step-and-repeat mode, whichever is optimal, in accordance with a layout of shot regions on the photosensitive substrate, or the like. Therefore, when, for example, mask patterns to be exposed (or shot regions on the photosensitive substrate) occupy an elongated area, the step-and-scan mode is adopted, while the step-and-repeat mode is adopted when the sensitivity of the photosensitive substrate is high and the exposure time is further reduced in the step-and-repeat mode. Thus, it is possible to make the most of the advantages of both the step-and-repeat method and the step-and-scan method fully.
On the other hand, the step-and-repeat mode in which the distortion characteristics are substantially fixed is adopted when the overlap exposure is effected by use of a single projection exposure apparatus and a high overlap accuracy (high alignment accuracy) is intended to be kept, while the scan-exposure mode in which a degree of luminance is enhanced with a slit-like exposure region is adopted. Thus, it is possible to make up for the disadvantages of the step-and-scan method and the step-and-repeat method.
When a direction of a stepping movement among the shot regions on the photosensitive substrate is switched over in accordance with a used exposure mode, the stepping is effected in a direction having a shorter moving distance, whereby there is an advantage that the stepping time can be shortened.
When a plurality of optical integrators are replaceably provided in the illumination optical system and these optical integrators are switched to be used in accordance with the exposure mode, even if an exposure mode is changed and the size of the exposure region on the photosensitive substrate is changed, deterioration in the illumination efficiency can be prevented.
It is still another object of the present invention to increase the size of an exposure field which can be transferred by one scan by using a projection optical system having a circular image field and utilizing a rectangular or slit-like region projecting along the diameter within said circular image field.
It is still another object of the present invention to control one or both of a first light-shielding means (shutter) which is disposed between a light source and a secondary light source generating means and a second light-shielding means (movable blade) which is disposed between the secondary light source generating means and a condensing optical system in cooperation in accordance with a sequence of the scan-exposure.
It is still another object of the present invention to accurately correct a relative rotational error (yawing) generated during a relative movement between the mask and the photosensitive substrate to improve the quality of an image which is projected onto the photosensitive substrate, as well as to improve an overlap accuracy between a pattern region formed on the photosensitive substrate and projected images of the mask patterns.